The present invention relates to proportioning or self-lapping type valve devices and particularly to such valve devices when arranged in railway brake systems to operate in either a direct release mode or a graduated release mode.
In U.S. Pat. No. 4,033,632, there is disclosed a proportioning type control valve capable of being operated in either a direct or graduated release mode by positioning a manual selector valve. In direct release mode, the valve operates on the pressure equalization principle and in graduated release mode, the valve operates on the pressure proportioning principle. Operation in graduated release mode is satisfactory, but in direct release mode, an overreduction of brake pipe pressure is required to obtain a full service brake pressure corresponding to the full service brake pressure obtained in graduated release mode. This is due to the fact that a relatively strong return spring is required to bias the smaller piston of a dual piston service valve assembly toward its release position in order to assure that during a subsequent brake application, movement of the larger piston of the service valve assembly will first compress the spring of a charging and dissipation valve, so that this valve can close before the larger piston movement is transmitted to the smaller piston, via the charging and dissipation valve, to in turn cause the smaller piston to actuate a supply valve via which pressure is supplied to the brake cylinder. The effect of the large return spring is compensated for in graduated release mode by the size ratio between the large and small pistons, with the brake pressure acting on the small piston and a constant reference pressure acting on the large piston. However, in direct release mode, the smaller piston is vented to atmosphere and the pressure on the larger piston is used as a supply pressure and is thus not constant. Accordingly, the bias effect of the return spring causes the service valve to prematurely assume a lap position in which the service valve terminates the supply of pressure to the brake cylinder displacement reservoir, so that a full service brake application in direct release mode is less than a full service brake application in graduated release mode.